The present invention relates to high-powered laser sources and, more specifically, the present invention relates to an apparatus for generating high-powered green and blue light lasers.
Lasers are widely used in a wide range of products involving digital audio and video media, telecommunications, remote sensing, electro-optic countermeasures, and other products. Because of the increasing demand for products utilizing lasers, the need for high-power lasers has increased over the years in some technology areas. It is also known that there is an increasing demand for blue and green light lasers, i.e., lasers having a wavelength in the 300 to 600 nm range. Conventionally, in order to provide a blue or green laser with increased power, it was necessary to increase the complexity and size of the laser, resulting in a substantial cost increase. Thus, an expensive laser that filled an entire room could provide a desired power level, but was infeasible for many applications.
In currently known optical systems, light propagation is commonly controlled by optics based on reflection and refraction. Many existing optical systems rely only on reflection and refraction to achieve the desired beam transformation. However, it is known that diffraction can also be used to achieve beam transformation in optical system design. In diffractive optics design, surface (two-dimensional) profiles can be designed to manipulate phase front of an optical beam in order to achieve the desired beam transformation. For example, diffractive optics can be designed to very effectively correct spherical aberration that is inherent to conventional lenses based on reflection and refraction. In addition, traditional lens designs based on reflection and refraction are based mainly on three-dimensional structures, which have a higher degree of complexity in the construction. The three-dimensional structure also makes the integration of the lens component into optical systems more difficult and less compact.
The present invention provides a system and method for generating a high-power blue or green light laser beam. The present invention utilizes a compact, low-cost design to produce a high-power, stable, single-mode output beam from an unstable, multi-mode diode laser source. More specifically, the high-power laser source is an infrared broad-area semiconductor laser having at least one watt of output power or a broad-area semiconductor laser bar or array having at least twenty watts of output power. The high-power multi-mode laser source is focused into a small area and directed into a nonlinear crystal through the use of diffractive optics. In one case, the light beam passing through the nonlinear crystal generates a blue laser light through frequency doubling and, in another case, generates a single mode longer wavelength laser light, which is converted to a green laser light through an alternative nonlinear crystal. The blue or green light is produced through frequency doubling in the nonlinear crystal. The use of diffractive optics effectively corrects longitudinal spherical aberrations created by the conventional lenses and, in addition, the diffractive optics effectively focus the output beams from broad-area semiconductor lasers, broad-area semiconductor laser bars or laser arrays. For the case of the high-power multi-mode laser source directly generating a blue or green laser light, diffractive optics are used to provide the necessary feedback to obtain a single-mode laser light prior to entering the nonlinear crystal for frequency doubling.
In one embodiment of the present invention, optical feedback devices are used for the modal shaping and pattern stabilization of high-output-power diode lasers. In various configurations, nonplanar surfaces are used for the modal shaping and generation of a single mode corrected laser beam. These nonplanar surfaces are diffractive optics, which include traditional grating devices, advanced digital and binary optics, and continuous surface relief diffractive optics. Traditional grating devices of both transmissive and reflective types are used as a feedback mechanism to the source laser to produce a stable, single-mode laser to be used with a nonlinear laser to produce a blue or green laser. One example of a transmissive diffraction grating is a volume holographic transmission grating. Examples of a reflective diffraction grating include linear and blazed reflection grating. Diffractive optics including digital/binary optics and continuous surface-relief diffractive optics can also be made to be either transmissive or reflective. The stable, single-mode beam created by the diffractive feedback is suitable for use in a ring resonator or other cavity designs utilizing a nonlinear crystal to generate a blue or green laser.
In another embodiment, diffractive optical elements including binary/digital optics or continuous surface-relief diffractive optics are provided for modal shaping and correction for cavities optimized for second harmonic. These elements have diffractive surfaces located directly on the laser crystal and/or cavity mirrors, resulting in a passive modulation or correction of the feedback in the laser cavity. The feedback can be used to optimize the modal characteristics of the cavity, which can lead to much greater second harmonic generation efficiency.
In another embodiment of the present invention, diffractive optics are used to focus the output beam of a high-power laser. Due to the nature of the emitting light beam of broad-area semiconductor lasers, i.e., highly divergent in one direction (over 40 degrees half angle) and less divergent in the second direction (approximately, 8 degrees half angle), the use of a conventional lens design to focus a light beam suffers greatly from longitudinal spherical aberration. In this embodiment, a diffractive optical device is attached to a plano-convex lens for correcting the longitudinal spherical aberrations created by the conventional lens. The surface of the diffractive optics can also be digitized into a digital/binary optics format. Furthermore, these surface profiles, digitized or not, can be attached directly on the surface of a conventional lens. Thus, the hybrid-focusing lens (coupler) created by the diffractive optical device and the plano-convex lens allows for higher power to be directed into a nonlinear crystal for second harmonic generation.
The above-described embodiments of the present invention comprise the following elements: a focusing lens (coupler) based on diffractive optics theory to effectively capture and direct the output light beam coming from a high-power, broad-area, infrared laser diode, and an optical feedback structure based on diffractive optical components to correct the quality of the laser beam. For the focusing lens (coupler), the design is accomplished by first obtaining the necessary phase changesxe2x80x94two different size collimated beams, corresponding to two different emitting directions, will need to go through to become focused. The difference of these two phase changes becomes the phase that should be provided by the diffractive optics for the laser light to be focused without longitudinal spherical aberration. The fabrication is accomplished through diffractive optical component fabrication techniques, such as gray-scale mask continuous surface-relief diffractive optics fabrication and photolithography binary optics fabrication techniques.
For the mode correction, the design of the present invention is based on the idea of providing an optical feedback that is not directly proportional to the original light distribution and, as a result, discouraging the possibility of stronger (weaker) signals receiving stronger (weaker) feedback and becoming even stronger (weaker). Therefore, any feedback device that allows for the redistribution of the amplitude or phase of the light can, theoretically, provide certain level of mode control. Under this design concept, the above-described components provide feedback that is not directly proportional to its original signal, which helps to eliminate the filamentation. It is further evidenced that the higher level the redistribution of the original signal that the device creates in the feedback, the higher capability the device possesses in discouraging filamentation. Diffractive components that allow the redistribution of the original light beam (when the light is directly reflected or the light is transmitted and then reflected by a mirror) can provide mode control of the output of the broad-area semiconductor lasers. Therefore, the mode-controlling component of the present invention may include any type of diffractive optical components, such as both amplitude gratings and phase gratings. Phase modulation in a phase grating is controlled through the change of the refractive index of a material, such as a volume grating, through the change of physical thickness of a material, such as a linear/blazed diffraction grating, or through a more advanced grating component, such as surface-relief diffractive optics and digital (binary) optics of both transmissive and reflective types. One example of a transmissive grating includes a holographic grating. The diffractive surface of these optical components is an effective beam-shaping and controlling device that is compact in size, and planar (two-dimensional) realization, thus, simplifying the manufacturing process of such a device.
In another embodiment of the present invention, the laser source comprises a diode laser bar and/or a diode laser array with an output of twenty watts or higher. These bars or arrays usually have an output width of 1 cm, versus about 100 micrometers of a broad area diode laser; therefore, capturing the light from these laser arrays is even more difficult than the broad-area diode lasers described above. Accordingly, the present invention is applied to control beams emitted from diode laser bar and/or a diode laser array so that they are suitable for use in either an end-pumped or side-pumped configuration. In these pump configurations, single-mode laser light at a longer wavelength is first produced via a gain medium; this single-mode laser light is then used to effectively produce a single mode blue or green laser light through a nonlinear crystal.
The present invention also involves novel designs for coupling pump radiation into a laser crystal. The couplers involve light ducts and concentrators of various profiles in addition to high reflective beam shapers. These couplers use planar, conic, and quasi-parabolic surfaces. The couplers are made of solid materials, hollow, or hollow with additional corrective elements. In addition to beam shaping, these couplers are also used to provide some control of the beam quality through manipulating the distribution of light intensity. The concept of diffractive optics is also employed to incorporate phase-correcting surfaces into the design of these couplers. This novel design can be utilized to optimize the modal properties of the laser cavity by modifying the crystal excitation profile and thus influencing the gain profile in the crystal. This is extremely important since the modal properties of the laser cavity are crucial to the success of achieving high second-harmonic output power.